The present invention relates generally to force measurement instrumentation, and more particularly to instrumentation adaptable to the special challenges of micromechanical applications.
The development of practical micromechanical devices which can be operated reliably and manufactured routinely and with high process yield is currently hampered by a virtual absence of standard diagnostic instrumentation. Such fundamental parameters as physical structure, displacement distance, spring constants, fracture strength, forces, and many others cannot at present be measured routinely. Relative values for such parameters can at times be inferred from operating voltage, measured capacitance, and the like, but such indirect estimates fail badly when absolute accuracy is needed.
Fracture and material fatigue are structural phenomena which limit the performance of micromechanical apparati. Fracture strength in particular appears to depend sensitively upon process variations. The ability to rapidly and conveniently measure the fracture behavior of micromechanical materials would greatly assist the design process, and also provide a useful process diagnostic.
On the micromechanical size scale (0.1 to 100 microns), most materials used or contemplated are quite strong by conventional standards. This is related to the relatively low level of structural defects present in thin films of most materials considered suitable for micromechanical applications, and to impediments to dislocation motion and multiplication presented by the size of micromechanical structures. Typical materials used or contemplated for use include polycrystalline silicon, silicon oxides, silicon nitride, crystalline silicon, metals such as aluminum, tungsten, and gold, amorphous and polycrystalline diamond, and so on.
Despite the high strengths typical of micromechanical materials, the fracture behavior is still limiting, owing too the huge material strains which can be encountered in microelectromechanical systems (MEMS). Operating strains as large as 0.01 can be found in MEMS devices. Induced strains of this magnitude interact strongly with any flaws, grain interfaces, and other imperfections in a structural material. As a result, apparently small changes in microstructure, as might result from normal process variations, can dramatically alter yield, fracture, and fatigue behavior. Direct measurement of such phenomena as a routine part of the fabrication process is therefore greatly to be desired.
The use of electrostatic forces to fracture a test MEMS element is of a beneficial approach, particularly if this test is to be routinely applied as a process diagnostic. MEMS devices often use electrostatic forces to drive their operation, and the similarity in the devices allows co-fabrication. Also, rather than having to attach any external mechanical elements for the fracture test, application of a voltage across a pair of terminals is sufficient. The result is a much simpler test to build and use. However, electrostatic forces are quite small, and previous techniques have not proven suitable for MEMS process diagnostics.
A common prior art approach is to apply a force to a cantilever beam, usually by mechanical transfer of an externally generated force, until the beam fractures. However, integral electrostatic drives do not provide sufficient force to produce such fracture. An external electrostatic actuator (e.g., an interdigitated electrostatic comb drive) can generate large enough forces, but such devices use an unacceptably large area on a process wafer to be used as a routine diagnostic.
An electrostatically driven resonance technique has been used to apply strain to a MEMS-like test structure. Near resonance conditions, the storage of vibrational energy within the test structure produces localized areas where the strain is much greater than would be applied statically by the same drive mechanism. The behavior of the resulting system, however, is complex and rather difficult to interpret. It also measures a fatigue failure condition, rather than a simple fracture condition, because the strain is applied at the resonance frequency. Finally, such devices have not been able to induce failure of the test structure without the prior introduction of a microcrack. The test thus gives no information about initiation of fracture, which is usually the limiting step in material failure.
Finally, for pure materials studies it is possible to create a series of MEMS test structures such that a high level of residual strain is induced by the fabrication process, the eventual result being fracture. However, large residual strains are incompatible with proper function of most MEMS devices. MEMS devices are quite sensitive to unexpected or inhomogeneous residual strains. As a result, even if residual strains can be well localized on a small testing device, the associated far-field induced stresses can easily be large enough to prevent proper MEMS device function elsewhere on the process wafer. Again, such procedures appear undesirable for MEMS process diagnostics.
Accordingly, there is a long-felt need for a micromechanical device which can measure absolute forces with reasonable accuracy. Ideally, such a device would be integrable with production microelectromechanical systems (MEMS), and could be calibrated independent of other MEMS devices. Further, interpreting the output of such a device would be simplified if the basic design required limited material strain for operation. Finally, real-time diagnostics for proper functioning of the device would be useful.
The invention is of a new class of instruments to measure fracture and fatigue properties of structural materials for micromechanical devices. Electrostatic forces applied normal to a rigidly mounted compliant testing membrane are therein converted into forces which stretch the membrane. A stress concentration structure which is part of the compliant testing membrane allows the static or repeated imposition of multi-GPa stresses on the material being tested, thereby making fracture and fatigue studies practical in the micromechanical size regime.